1. Field of the Invention
This invention relates to methods for solid state lighting and in particular compact monolithic solid state lamps comprising multiple lighting elements.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
In order to use an LED chip in a circuit or other like arrangement, it is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and the like. An LED package also includes electrical leads, contacts or traces for electrically connecting the LED package to an external circuit. In a typical LED package 10 illustrated in FIG. 1a, a single LED chip 12 is mounted on a reflective cup 13 by means of a solder bond or conductive epoxy. One or more wire bonds 11 connect the ohmic contacts of the LED chip 12 to leads 15A and/or 15B, which may be attached to or integral with the reflective cup 13. The reflective cup may be filled with an encapsulant material 16 which may contain a wavelength conversion material such as a phosphor. Light emitted by the LED at a first wavelength may be absorbed by the phosphor, which may responsively emit light at a second wavelength. The entire assembly is then encapsulated in a clear protective resin 14, which may be molded in the shape of a lens to collimate the light emitted from the LED chip 12. While the reflective cup 13 may direct light in an upward direction, optical losses may occur when the light is reflected (i.e. some light may be absorbed by the reflector cup due to the less than 100% reflectivity of practical reflector surfaces). In addition, heat retention may be an issue for a package such as the package 10 shown in FIG. 1a, since it may be difficult to extract heat through the leads 15A, 15B.
A conventional LED package 20 illustrated in FIG. 1b may be more suited for high power operations which may generate more heat. In the LED package 20, one or more LED chips 22 are mounted onto a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23. A metal reflector 24 mounted on the submount 23 surrounds the LED chip(s) 22 and reflects light emitted by the LED chips 22 away from the package 20. The reflector 24 also provides mechanical protection to the LED chips 22. One or more wirebond connections 11 are made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The mounted LED chips 22 are then covered with an encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens. The metal reflector 24 is typically attached to the carrier by means of a solder or epoxy bond.
Typical LED components for solid state lighting applications attempt to achieve high light output by operating single LED chips at as high as possible current and at a low voltage typical for individual LEDs. For higher powered operation it may also be difficult to transfer dissipate heat generated by the LED chip 22. Submounts 23 can be made of materials such as ceramics that are not efficient at conducting heat. Heat from the LED chip passes into the submount below the LED chip, but does not efficiently spread laterally from below the LED. This increased heat can result in reduced lifetime or failure of the package.
At the systems level high current operation necessitates relatively expensive drivers to provide the constant DC current source for such components. Operating SSL components at lower currents and higher voltages instead would provide for lower cost driver solutions and ultimately lower system costs. This is currently achieved by assembling multiple LED components of a suitable current rating in series at the circuit board level. The lower driver cost for such solutions is outweighed by the high cost of the individual components.
Current LED packages (e.g. XLamp® LEDs provided by Cree, Inc.) can be limited in the level of input power and for some the range is 0.5 to 4 Watts. Many of these conventional LED packages incorporate one LED chip and higher light output is achieved at the assembly level by mounting several of these LED packages onto a single circuit board. FIG. 2 shows a sectional view of one such distributed integrated LED array 30 comprising a plurality of LED packages 32 mounted to a substrate or submount 34 to achieve higher luminous flux. Typical arrays include many LED packages, with FIG. 2 only showing two for ease of understanding. Alternatively, higher flux components have been provided by utilizing arrays of cavities, with a single LED chip mounted in each of the cavities. (e.g. TitanTurbo™ LED Light Engines provided by Lamina, Inc.)
These LED array solutions are less compact than desired as they provide for extended non-light emitting “dead space” between adjacent LED packages and cavities. This dead space provides for larger devices, and can limit the ability to shape the output beam by a single compact optical element like a collimating lens or reflector into a particular angular distribution. This makes the construction of solid state lighting luminares that provide for directed or collimated light output within the form factor of existing lamps or even smaller difficult to provide. These present challenges in providing a compact LED lamp structure incorporating an LED component that delivers light flux levels in the 1000 Lumen and higher range from a small optical source.
Current high operating voltage luminaire solutions integrate multiple discrete LED components as assemblies at the circuit boards level. To achieve the desired beam shape individual optical lenses are mounted with each LED component, or very large reflectors (larger than the form of existing conventional sources) have to be employed. These secondary optical elements (lenses or reflectors) are large and costly, and the extended area of such single chip arrays further provides for a more expensive LED luminaire. Additionally, any light being reflected from the sidewalls in the packages and cavities can also result in additional optical losses, making these overall LED components less efficient.